This invention relates to the field of logic circuits, more particularly to the area of bipolar transistor logic circuits utilizing push-pull output drive stages.
Present push-pull output drive stages of bipolar transistor logic circuits comprise a dual transistor arrangement wherein an upper transistor is coupled between a DC voltage supply and an output load and a lower transistor is coupled between the output load and ground. In operation a high output voltage is realized at the output terminal by turning on the upper transistor and turning off the lower transistor; a low output voltage is realized by turning off the upper transistor and turning on the lower transistor; a high impedance or third state is achieved by turning off both transistors.
It is highly desirable to be able to switch from one output state to the other as fast as possible. In general, increasing the switching speed involves the dual problems of removing the excess stored base charge from one of the output transistors and injecting relatively large amounts of base current into the other output transistor. During the high-to-low transition time or fall time of the output voltage, the upper output drive transistor coupling the power supply to the load must turn off. That is, a charge stored in the base must be removed, and any stray base current such as that coupled through the intrinsic collector-to-base capacitance must also be drained away from the transistor. The lower output drive transistor which couples the load to the ground must, on the other hand, be supplied with a large amount of base current in order to turn the transistor on to in turn provide a low impedance to the load at the output terminal.
Some prior art conventional circuits have used diode coupling from the load back to the base of the lower output drive transistor to supply base current and thereby improve the fall time. However, diode coupling to supply base current is not compatible with a three-state output logic circuits in which the third output state or high impedance state is available along with the normal binary output states. These type of circuits are used in systems wherein several logic circuits share a common output bus as is commonly found in computer architecture. The result is that present three-state transistor logic circuits have no additional circuitry to aid the fall time.
Accordingly, an object of this invention is to improve the fall time in bipolar transistor logic circuits.
It is also an object of this invention to provide additional base drive to the lower output drive transistor during the high to low transition of the output voltage.
It is another object of this invention to remove stored base charge and stray base current due to collector-base capacitance in the upper output drive transistor during high to low transition of the output voltage.
It is still another object of this invention to provide a base drive boost circuit which is compatible with three-state transistor logic circuits.
It is another object of this invention to provide a base drive boost circuit which does not draw current from a DC power supply.
It is still another object of this invention to supply base drive boost current which is proportional to the load capacitance at the output terminal of the circuit.
It is another object of this invention to provide a second path for sinking load current during the fall time of the output.